1. Field of the Invention
The present invention generally relates to timebase error correcting apparatus (i.e., timebase corrector) and, more particularly, is directed to a timebase corrector having a velocity error correction function and which is suitable for use with a home video tape recorder (VTR).
2. Description of the Prior Art
In most of the home video tape recorders now commercially available on the market, a color signal down converting system is employed, in which a color signal is down converted and the down converted signal is re-converted to the signal of original band by a heterodyne system in the playback mode. A timebase fluctuated component of a chroma signal is removed by a double heterodyne method utilizing a quartz oscillator and a timebase fluctuated component of a luminance signal is corrected by an automatic frequency control (AFC) circuit in a horizontal scanning circuit in a television receiver so that a timebase corrector is not needed generally. On the other hand, in a professional video tape recorder of direct color process system (e.g., one-inch helical scan type video tape recorder, a 4-head video tape recorder and so on), a timebase corrector is frequently used because a luminance signal and a chroma signal are not separated and also an interleave relationship must be maintained therebetween.
An example of a conventional timebase corrector will be explained with reference to a schematic block diagram forming FIG. 1.
Referring to FIG. 1, a video signal to be timebase-corrected is applied to an input terminal 1, and is fed to an analog-to-digital (A/D) converting circuit 2 and a write clock generating circuit 3. The write clock generating circuit 3 generates a write clock which is coincident with a timebase fluctuation involved in the video signal. The luminance signal component having the timebase fluctuation is sampled by the write clock, and the analog video signal is converted into a digital video signal by the A/D converting circuit 2, that is, the analog video signal is pulse code modulated (PCM-modulated) and this PCM video signal is written in a digital memory 4. The digital video signal written in the digital memory 4 is read out by a read clock from a read clock generating circuit 6 to which a reference synchronizing (sync.) frequency is applied, and the thus read-out video signal is reconverted into the analog video signal by a digital-to-analog (D/A) converting circuit 5 to which the read clock is supplied, thereby a video signal whose timebase fluctuation is stabilized being obtained at an output terminal 7.
On the other hand, in the video tape recorder, the clamping and the switching of video heads are performed by utilizing a blanking period (i.e., vertical blanking period) of the video signal. In the case of an NTSC television composite video signal, a vertical synchronizing pulse 8 exists ahead of and behind an equalizing pulse 3H in this blanking period as shown in FIG. 2, and this vertical synchronizing pulse 8 is employed as a reference timebase in the vertical direction of the picture screen.
As described in the example of the prior art, the timebase corrector is not generally employed by the home video tape recorder unlike the professional video tape recorder. Even in the home video tape recorder employing the timebase corrector, if residual error involving the vertical synchronizing pulse is largest when a color burst signal is taken as the reference point of the timebase correction, unlike the professional video tape recorder, it is very difficult to reliably detect a vertical synchronizing signal from a reproduced signal in which a waveform distortion, a skew distortion, a drop-out or the like occur frequently. There is then the large possibility that the reference timebase in the vertical direction of the picture screen will be dropped out. More specifically, if a drop-out, noise 9 or the like enters in the vicinity of the vertical synchronizing pulse 8 during the blanking period as shown in FIG. 2, then the detection of the vertical synchronizing pulse 8 becomes difficult and a picture is disturbed in the vertical direction, deteriorating the image quality considerably.
Further, Japanese Patent Laid-Open Gazette No. 63-194494 describes a color video signal recording method. In this color video signal recording method, first and second color signals of a color video signal, which is composed of a luminance signal and first and second color signals, are converted in a line sequential fashion, timebase-compressed to 1/4 and then multiplexed on the luminance signal. Then, by timebase-expanding the multiplexed signal, that is, a color line sequential TCI (time compressed integration) signal about twice so as to be divided into two channel signals and scanning one field period of the two channel signals twice or three times, the color video signal is simultaneously recorded in two slant tracks.
Japanese Patent Application No. 2-51248, which is not yet laid open when the assignee of the present application files this application describes the above-mentioned recording method, a recording apparatus, therefor, a reproducing apparatus thereof, a timebase error correcting apparatus (TBC) provided in the reproducing apparatus and so on.
Examples of the conventional video tape recorder for recording a color video signal on a magnetic tape by the above-mentioned recording method, reproducing the recorded signal from the magnetic tape and a timebase corrector thereof will be explained.
Initially, first and second color signals of a color video signal composed of a luminance signal and first and second color signals (red and blue color difference signals) are converted in a line sequential manner, timebase-compressed to 1/4, multiplexed with the luminance signal and the multiplexed signal (i.e., TCI signal) is timebase-expanded to about twice, while a 2-channel signal, obtained by a shuffling process, is supplied to an FM modulator circuit, in which it is FM modulated.
Incidentally, two sets or four of rotary magnetic heads are mounted on a rotary drum of a tape guide device in a close relation and head gaps of the rotary magnetic heads mounted closely to each other are different in azimuth angle. Further, azimuth angles of head gaps of four rotary magnetic heads are selected such that, when four rotary magnetic heads are rotated one turn, the recording azimuth angles of four slant tracks formed on the magnetic tape are sequentially changed as +.theta., -.theta., +.theta. and -.theta..
The timebase-expanded TCI signal of one field is recorded such that two sets of slant tracks of two channels are formed on the magnetic tape per revolution by sequentially supplying the FM modulated signals of two channels to the two sets of rotary magnetic heads, and accordingly, the timebase expanded TCI signal of one frame is recorded so as to form four sets of slant tracks of two channels per two revolutions.
In the magnetic tape on which the signal is recorded as described above, signals of two channels on the magnetic tape are reproduced by two sets of another rotary magnetic heads having the same head arrangement and gap azimuth angles as those of the rotary magnetic heads in the recording mode. The reproduced signals of two channels are supplied to and demodulated by respective FM demodulator circuits, demodulated signals are converted in the form of analog to digital signals, written in respective timebase error correcting memories and read out therefrom to thereby correct the timebase error.
A write clock signal containing a jitter, generated on the basis of a horizontal synchronizing signal and a burst signal separated from the FM demodulated timebase expanded TCI signal, is supplied to the two memories upon writing, while a read clock signal obtained from a fixed oscillator is supplied to the two memories upon reading.
The two channel signals from the respective memories are supplied to and decoded by a common luminance signal decoder and a common chrominance signal decoder, that is, luminance signals and line-sequential chrominance signals are extracted respectively from these two channel signals, the luminance signals and the line-sequential chrominance signals are timebase compressed by about a half and the line-sequential chrominance signals are timebase expanded by about four times, converted to simultaneous chrominance signals, and resultant digital luminance signals and digital red and blue color difference signals are converted in the form of digital to analog signal, thereby original luminance signal and red and blue color difference signals being obtained.
On the basis of the horizontal synchronizing signal and the burst signal respectively separated from the FM demodulated and timebase expanded TCI signals of two channels, velocity error signals are generated respectively, and these velocity error signals are stored in memories exclusively used for matching their times with the TCI signals read out from the timebase error correcting memories, the clock signal obtained from the fixed oscillator is phase-modulated by the velocity error signal read from the memory to thereby generate a read clock signal and this read clock signal is supplied to the timebase correcting memory and the D/A converter.
In such conventional timebase error correcting apparatus for a video tape recorder, two sets of circuits are needed in order to correct the timebase errors and velocity errors of the pair of reproduced video signals which result from simultaneously reproducing video signals respectively recorded on the pair of slant tracks on the magnetic tape by the pair of rotary magnetic heads. Thus, the circuit arrangement of the prior-art timebase corrector becomes complicated and the consumption of power is considerably increased.